During the past thirty years over 1.4 billion pounds of polychlorinated biphenyls (PCBs) have been produced in the United States alone. Increasing public awareness and concerns of PCB contamination and toxicity to animals and man have prompted the Environmental Protection Agency (EPA) to regulate its disposal and to require the phasing out of its use. How to dispose of PCBs is only one of a number of toxic waste problems to be faced in the very near future. Current EPA estimates, based on 1981 figures, indicate an annual hazardous waste generation of seventy-one billion gallons. Hazardous wastes include such unattractive materials as pesticides, herbicides, hospital wastes including pathogens, paints, inks and contaminated solvents, black liquor, and explosives. These wastes will all require destruction at some point in their use cycle.
The following reactor types have been offered or used commercially as solutions to hazardous waste disposal: rotary kiln, liquid injection, multiple hearth, multiple chamber, fluidized bed, molten salt, and high efficiency boilers. Though each of these reactors has some desirable practical and theoretical operating features, none can claim profitable operation. Of the above reactors, the rotary kiln is most commonly employed in the United States and Europe. Although such reactors can achieve a combustion efficiency of greater than 99.99 percent, post-combustion and high residence times are typically required. Incineration of this type involves a combination of pyrolysis (1200.degree. C. and combustion that is initiated by a high temperature flame. Although the initial pyrolysis transforms the organic compound into a more oxidizable form, the oxidation process requires the actual collision of the resulting incipient high energy fragments with oxygen. In a rotary kiln, where the reaction medium may tend to be highly viscous, it is often difficult to bring the reacting species into direct contact with oxygen. This lack of efficient mixing on a molecular level impedes the rate of destruction. Rotary kilns are therefore typically inefficient, requiring excess oxygen and, hence, more auxiliary fuel. The actual exothermic (heat liberating) reaction, attending the reaction with oxygen, occurs away from the flame tip as the reacting materials are fed through the reaction chamber. Consequently, the heat generated by these reactions cannot be utilized efficiently for the initial endothermic pyrolysis step.
A high-turbulence combustion chamber having a pulsating spiral flow will lower the residence time and temperature required for destruction, according to Rathjen et al in U.S. Pat. No. 4,140,066.
We are also aware of the work of certain others in the field of coal gasification which may be relevant to a consideration of prior art processes. In Rummel's U.S. Pat. No. 2,647,045, for example, a molten slag bath obtained from the reduction of iron ore or from the "non-combustible residues of coal products" is circulated and finally divided coal is injected into the bath and a separate addition of air is also conducted along with "an endothermic gaseous reactant", i.e. water. The process is preferably conducted in two separate endothermic a exothermic zones. Thus, the elementary idea of using the latent heat of molten slag to combust coal is known. Rummel made certain improvements and variations of his basic approach, as disclosed in U.S. Pat. Nos. 2,848,473; 2,923,260, and 3,953,445, none of which enhance the relevance of the basic idea to the present disclosure. An iron bath is used for coal gasification in U.S. Pat. No. 4,388,084. In U.S. Pat. No. 4,389,246 to Okamura et al, on the subject of coal gasification employing a molten iron bath, the bottom-blowing of ethane is described (see particularly column 6, lines 7-14); the ethane or other hydrocarbon gas is used to stir the mixture and for this reason is considered by Okamura et al to be equivalent to oxidizing gases and inert gases as well as oxidizable gases.
Injection from above is also employed in Gernhardt et al U.S. Pat. No. 4,043,766, Okamura et al U.S. Pat. No. 4,389,246, Okane et al U.S. Pat. No. 4,388,084, and Bell et al U.S. Pat. No. 4,431,612.
Titus et al, in U.S. Pat. No. 3,812,620, envision a molten pool of glass and miscellaneous metals obtained during the incineration of "heterogeneous waste materials" such as municipal garbage; the various organics are "decomposed" in the pool at temperatures of the order of 1600.degree. F. and "further pyrolyzed" ("at least some gases") at 2000.degree. F. While the inventors (in column 5) mention the possibility of temperatures of up to "10,000.degree. F. or more" in order to ensure that iron remains in a molten state, they do not add oxygen in the bath and appear to utilize the bath only for the thermal decomposition of miscellaneous organics. See also von Klenck et al U.S. Pat. No. 3890,908. Yosim et al, in U.S. Pat. No. 3,845,190, also envision pyrolytic destruction in a bath followed by oxidation in a zone above it.
The molten salt process involving the reaction of material with a hot alkali metal is typified by the disclosure of Grantham's U.S. Pat. No. 4,246,255 maintaining a bath of, for example, alkali metal carbonates, at about 700-1000.degree. C. Oxygen is also injected into the molten salt. Southwick, in U.S. Pat. No. 3,527,178, employs a metal bath.
Molten iron is employed by Rasor in U.S. Pat. Nos. 4,187,672 and 4,244,180 as a solvent for carbon generated through the topside introduction of coal; the carbon is then partially oxidized by iron oxide during a long residence time and partially through the introduction of oxygen from above. The Rasor disclosure maintains distinct carbonization and oxidation chambers.
We are also aware of the relatively large-scale destruction of PCBs in large utility boilers through their addition to the conventional fuel in amounts up to 5%. See "Destruction of High Concentration PCBs in a Utility Boiler" by Siedhoff, Zale & Morris, proceedings of the 1983 PCB Seminar, Electric Power Research Institute. While this appears to be an expedient disposal method, and the destruction of PCBs reaches the EPA requirement of over 99.99 percent, the long-term corrosion and other effects on the high-efficiency boiler are largely unknown; likewise the oxidation cannot be as efficient as that in our own process, and in fact the handling costs for the PCBs tend to equal or exceed the fuel value.
Profitable decomposition of hazardous waste must rely heavily on efficient, cost-effective recovery of energy and high-pury by-products. Generally, the economic recovery of high grade energy will be maximized when destruction occurs efficiently without auxiliary fuel addition.